The overall goal of the project is understanding the roles of proto- oncogenes in gametogenesis and early development of mammalian embryos. The proposed research is focused on the mechanisms that regulate transcription of the c-mos gene, which plays a critical role in meiosis of vertebrate oocytes. Transcription of c-mos is subject to stringent tissue--specific regulation, leading to its specific expression in male and female germ cells. Such regulation of c-mos is needed not only to achieve appropriate c-mos expression in germ cells but also to prevent inappropriate expression in somatic cells, which can result in either cell death or neoplastic transformation. It is also noteworthy that c- mos expression is downregulated early in embryogenesis, which may be necessary to prevent arrest of embryonic cleavage divisions resulting from the action of Mos as a cytostatic factor. Transcription of c-mos in somatic cells is suppressed by a negative regulatory element (NRE) upstream of the c-mos promoter. In addition, recent studies have identified a candidate somatic cell repressor that binds to a functional element within the c-mos NRE. We now plan to isolate a cDNA clone of the repressor protein and to characterize its mechanism of action in suppressing transcription of c-mos, and possibly other germ cell-specific genes, in somatic cells. Expression of c-mos in oocytes, which is not affected by the NRE, requires only a minimal promoter, including an initiator (Inr) element. These findings suggest the hypothesis that c-mos transcription in oocytes results from a high level of basal transcription activity, which is then suppressed in two-cell embryos. This will be investigated by further studies of the activity of the c-mos Inr in directing transcription in oocytes and embryos, together with analysis of the expression and function of the c-mos repressor during embryonic development. The proposed studies have three specific aims: 1. Isolation and characterization of a cDNA clone for a somatic cell repressor of c-mos transcription. 2. Structure/function analysis and studies of the mechanism of c-mos repressor action. 3. Investigation of the regulation of c-mos transcription in oocytes and early embryos.